Lex mercatoria is the Latin expression for a body of trading principles used by merchants throughout Europe in the medieval period. Meaning literally "merchant law", it evolved as a system of custom and best practice, which was enforced through a system of merchant courts along the main trade routes. It functioned as the international law of commerce.[1] It emphasised contractual freedom, alienability of property, while shunning legal technicalities and deciding cases ex aequo et bono.
A distinct feature was the reliance by merchants on a legal system developed and administered by them. States or local authorities seldom interfered, and surrendered some of the control over trade within their territory to the merchants. In return, trade flourished under the lex mercatoria, increasing tax revenues.

Law French is an archaic language originally based on Old Norman and Anglo-Norman, but increasingly influenced by Parisian French and, later, English. It was used in the law courts of England, beginning with the Norman Conquest by William the Conqueror. Its use continued for several centuries in the courts of England.

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Survivals in modern legal terminology

The inverted syntax of many legal noun phrases in English—attorney general, fee simple—is a heritage from Law French. Native speakers of French may not understand certain Law French terms not used in modern French or replaced by other terms: for example, the current French word for "mortgage" is hypothèque. Many of the terms of Law French were converted into modern English in the 20th century to make the law more understandable in common-law jurisdictions. However, some key Law French terms remain, including the following:

attorney, one appointed to act for another — now characterized as either:

attorney-at-law — see lawyer, solicitor, barrister or civil law notary

attorney-in-fact — one who has power of attorney.

autrefois acquit, a peremptory plea that one has previously acquitted of the same offense.

bailiff, the marshal of the court, charged now chiefly with keeping order in the courtroom.

cestui que trust, sometimes shortened to cestui; the beneficiary of a trust.

culprit, now used to mean 'guilty party'. Originally a blending of Latin culpabilis ('guilty') and Law French prist ('ready'), a shortening of a conventional phrase prist del averer ('[I am] ready to prove [that the accused] is guilty as stated').

cy-près doctrine, the power of a court to transfer the property of one charitable trust to another charitable trust when the first trust may no longer exist or be able to operate.

defendant, the party against whom a civil proceeding is brought.

escheat, reversion of unclaimed property to a feudal lord, or the state where the property is allodial.

estoppel, prevention of a party from contradicting a position previously taken.

feme covert and feme sole - the legal status of adult unmarried women and married women, respectively, under the coverture principle of common law.

force majeure - French for "superior force"; clause in some contracts that frees parties from liability for acts of God.

laches - loss of rights through failure to act.

mortgage - literally a "dead pledge"; a pledge by which the landowner remained in possession of the property he staked as security.

Statutes of Mortmain - statute restricting the conveyance of land to the "dead hand" of a religious organization

oyez - often calqued as hear ye!, a traditional cry used to open court proceedings, still used in the Supreme Court of the United States.

parol evidence rule, a substantive rule of contract law which precludes extrinsic evidence from altering the terms of an unambiguous fully expressed contract; from the Old French for "voice" or "spoken word," i.e., oral, evidence.

parole - from the Old French for "voice" or "spoken word"; the release of a prisoner based on giving their word of honor to abide by certain restrictions.

plaintiff - the person who begins a lawsuit.

pur autre vie - Pour autre vie in modern French; means during the term of another person's life (used in lease arrangements)

prochein ami - Law French for what is now more usually called next friend. Refers to one who files a lawsuit on behalf of another not capable of acting on his or her own behalf, such as a minor.

profit a prendre - also known as the right of common, where one has the right to take the "fruits" of the property of another, such as mining rights, growing rights, etc.

recovery - originally a procedural device for clarifying the ownership of land, involving a stylised lawsuit between fictional litigants.

remainder - originally a substitution-term in a will or conveyance, to be brought into play if the primary beneficiary were to die or fail to fulfill certain conditions.

replevin, a suit to recover personal property unlawfully taken.

torts, meaning wrongs.

trove, as in treasure trove, is an adjective, not a noun, and means found. Thus treasure trove means not a treasure chest or hoard, but a treasure found by chance, as opposed to one stolen, inherited, bought, etc. Trove should properly be a word of two syllables (Old French trové, modern French trouvé), but this is never observed today.

voir dire - literally "to say the truth"; the word voir (or voire) in this combination comes from Old French and derives from Latin verum, "that which is true" and is not related to the modern French word voir, which derives from Latin vidēre ("to see"). Voir dire refers to the questions a prospective juror or witness must answer to determine his qualification to serve, in the law of England a mini-trial held after a plea of guilty has been entered to determine the facts of the offence where they are in dispute. In a modern context thought of often as a mini-trial within a full trial to determine the admissibility of contested evidence. In a jury trial a voir dire is held before the judge but without a jury present. Voir dires may also be held in a trial by judge alone, but done, of course, in the presence of the judge.